I want to iterate over a function g with iterate. When I run my code the loop runs infinitely. Is there any possibility to get out of the loop when a condition is met?
Something like if g x > 100 then stop.
My code:
f x = iterate g x
g x = 2 * x
Just flip the condition, and takeWhile it holds:
f x = takeWhile (<= 100) $ iterate g x
Which could also be made point free:
f = takeWhile (<= 100) . iterate g
This will take from the infinite list that iterate returns while the accumulator is less than or equal to 100.
Yes, you either use takeWhile (not . p) for that, equivalent to
foldr (\x r -> if (not (p x)) then (x:r) else []) []
or sometimes the following is useful,
takeUntil p = foldr (\x r -> if (not (p x)) then (x:r) else [x]) []
if you want to include the last element into the sequence, as well.
In your case, you'd write takeWhile (not . (> 100)) . iterate (*2), or takeUntil (> 100) . iterate (*2).
(not . (> 100)) is of course the same as (<= 100).